Control methods for a direct-ground cooling system: An experimental study on office cooling with ground-coupled ceiling cooling panels

doi:10.1016/j.enbuild.2019.05.049

Energy and Buildings

Volume 197, 15 August 2019, Pages 47-56

https://doi.org/10.1016/j.enbuild.2019.05.049 Get rights and content

Abstract

This article experimentally investigates the performance of two control methods (the supply water temperature method and the water flow control method) for a direct-ground cooling system. The control methods were implemented through three types of indoor feedback controllers: 1) an on/off controller, 2) an on/off controller with a deadband and 3) P controller. The performances of the control methods were evaluated regarding room temperature stability and pump energy use. Ceiling cooling panels were employed to keep the test room air temperature at 25.0 °C under periodic heat gain conditions. The cooling system used a ground heat exchanger with a U-pipe in an 80-meter-deep borehole as a cooling source. The findings show that room air temperature was maintained close to the set-point with both control methods, but it was more stable with the temperature control method. In addition, condensation risk was easily preventable with the temperature control method. A parametric study showed that pump energy use when using two-position controllers could be as low as when using modulating controllers if the on-state flow rate of the circulating pump is selected in relation to the ground temperature and heat transfer characteristics of the heat exchanger.

Introduction

Cooling water in traditional systems for comfort cooling, e.g., for commercial buildings, is done using chillers or district cooling systems. These systems are often designed for relatively low temperature levels (4–8 °C temperature supply) with varying return temperatures depending on the application, e.g., air- or water-borne cooling systems. However, cooling can also be provided by employing “free-cooling” sources such as ambient air, sea or lake water, and ground sources. The disadvantage of employing free-cooling sources is that their temperature levels are rather high, so their source temperatures may not be fully compatible with the designed temperature levels of traditional building terminals.

There is, however, an increasing interest in developing the use of free-cooling sources to reduce the use of electric-driven chillers. This development requires higher (closer to room) temperatures in the cooling systems, which principally entails increasing heat transfer areas in the building [1]. One viable free-cooling source is ground. Due to its high thermal mass, ground at around 10 m deep has a rather stable temperature all year. The ground-cooling concept has already been successfully implemented in office buildings [2], [3], [4]. However, the cost for the necessary ground heat exchanger varies with the ground conditions.

This article is part of a research project that aims to develop the design and control of direct-ground cooling systems for office buildings. In direct-ground cooling systems, the terminal units are cooled by circulating water in an array of ground heat exchangers, typically U-tubes in water-filled boreholes in rock or in bentonite-filled boreholes in the ground. Thus, the minimum supply temperature of the building terminals is as high as the ground temperature. Hydronic radiant cooling systems (as described later in this section) are suitable for this application [5]. Hydronic convective systems, e.g., chilled beams and fan-coil units, are also suitable due to their enhanced heat transfer characteristics.

Radiant systems utilize circulating water to control the temperature of indoor surfaces, e.g. walls, floors and ceilings [6]. These systems are generally classified as three types: ceiling panels, embedded surface systems and thermally active building systems [7]. Controlling thermally active building systems may require sophisticated control systems because of the time lag between the operating modes, which is due to the large thermal mass involved. On the contrary, controlling low-mass systems, i.e., ceiling panels and embedded surface systems, may be implemented using conventional control methods for hydronic systems [8]. Studies regarding the capacity control of low-mass radiant systems have been performed based on the concept of supplying energy to the cooling system at the rate at which the energy is exchanged with the ambient [9]. The main control methods for these systems are heat flux-modulation, water temperature control and water flow controls [10], [11]. These control methods can be applied to the hydronic system by means of different controller types, valves, pumps, etc. These control methods have shown different performances in terms of controllability, energy demand of the system and room temperature stability [10], [11], [12], [13], [14], [15], [16], [17].

In ground cooling systems, the cooling capacity control of a building's cooling terminal units is usually performed by regulating the fluid rate in the ground loop and/or in the building loop. Adjusting the fluid rate in the ground loop (source) while keeping constant flow in the building loop causes temperature changes in the building loop. Likewise, adjusting the flow rate in the building loop while circulating a constant flow in the ground loop would change the inlet and outlet temperatures of the ground. This temperature interaction between the two loops influences the thermal performance of the ground and the terminal units in the building. Therefore, proper capacity control of building terminal units in ground cooling entails understanding which parameters are effective and to what extent.

To the best of the authors’ knowledge, no study has been conducted regarding control methods for ground cooling with concern to the thermal performance of the ground and ceiling cooling panel systems under periodic space heat gain. This study experimentally investigates the performance of two competitive control methods for direct ground-coupled ceiling cooling panels, i.e., supply water temperature and water flow control methods. The control methods were implemented using two-state controllers (on/off with and without a deadband) and a modulating controller (proportional controller). The main evaluation criterion for the methods was their capability to maintain the room air temperature. In addition, we also studied how water temperature and flow rate in the ground and building loops influences the controllability of the system.

Section snippets

Classification of control parameters

Different control methods have been developed and tested to efficiently operate radiant cooling and heating systems. These control methods are classified based on the variables they use as their input variable, controlled variable and manipulated variable [8], [10], [18]. The input variable, also known as set-point, is the reference variable used by the controller to adjust the cooling capacity of the terminal unit based on the changes in the process. Room air/operative temperature, surface

Experimental facility

The experiments were carried out in a test room that was 4.2 m × 3.0 m × 2.4 m (L × W × H). The room was built and outfitted as an office room (Fig. 1). The room was constructed using lightweight polystyrene panel walls with gypsum board finishing and lightweight false ceiling panels externally insulated with glass wool sheets on the topside. The test room was completely surrounded by an outer chamber, the temperature of which could be maintained at any intended temperature.

Fig. 2 shows the

Control methods and room temperature

Fig. 6 shows the room air and operative temperatures under periodic heat gain in the test room. As previously mentioned, the cooling capacity of the panels was controlled by either the temperature control method or the flow control method. The room air temperature set-point was 25.0 °C for the on/off controller and the P controller and 25.0 ± 0.5 °C for the on/off controller with a deadband. As Fig. 6 shows, the room air temperature was maintained close to the set-point in both control methods.

Evaluation of the control methods

This study aims to evaluate two control methods (the flow control method and the supply temperature control method) for ground cooling ceiling panels. The main evaluation criterion for the control systems was their capability to control the cooling capacity and have a stable room air temperature under periodic space heat gain. Although both methods performed similarly in controlling the room temperature, the room temperature fluctuations were moderately lower with the temperature control

Conclusion

We suggest adapting the temperature control method for direct-ground cooling ceiling panels because the room air temperature was maintained at the set-point with slightly smaller variations than that of the flow control method. The heat flux from the ceiling panels to the room was proportional to the supply temperature. Therefore, supply temperature control was a direct and linear method for controlling the cooling capacity of the panels. In addition, the condensation risk is easily preventable

Conflict of interest

None

Acknowledgements

This work was financially supported by the Swedish Energy Agency through its national research program EFFSYS EXPAND and by the in-kind contribution of laboratory facilities by Uponor. We are particularly grateful to Håkan Larsson for his lab assistance. Valuable discussions by Jonas Gräslund (Skanska), Peter Filipsson (CIT Energy management) and Qian Wang (Uponor) are gratefully acknowledged.

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Control methods for a direct-ground cooling system: An experimental study on office cooling with ground-coupled ceiling cooling panels

Abstract

Introduction

Section snippets

Classification of control parameters

Experimental facility

Control methods and room temperature

Evaluation of the control methods

Conclusion

Conflict of interest

Acknowledgements

Energy Build.

Sol. Energy

Build. Environ.

Energy Build.

Energy Build.

Build. Environ.

Build. Environ.

Earth-coupled radiant heating and cooling system for hot, humid climates

ASHRAE Trans.

Field measurements of operative temperatures in buildings heated or cooled by embedded water-based radiant systems

ASHRAE Trans.

Temperature and Humidity Independent Control (THIC) of Air-Conditioning System

Low temperature heating and high temperature cooling

Rehva

ASHRAE Handbook - HVAC Systems and Equipment

The temperature stability of a radiant slab-on-grade

ASHRAE Trans.

A study on the control methods for radiant floor heating and cooling system in residential building

ASHRAE Trans.

Temperature regulation of radiant floor heating systems using two-parameter on-off control: An experimental study

ASHRAE Trans.

Some control strategies for radiant floor heating

Radiant Times